How to make a round workpiece with a four-roll plate rolling machine

  • Jack Lu 
4 roller rolling machine

WHAT IS A 4-ROLL BENDING MACHINE?

The four-roll bending machines have over the 3-roll bending machines is simplicity.

Four-roll bending Machines produce the fastest and most accurate bends. It has fixed-position top rolls and two offset, or lateral, rolls, one on each side, and has an additional roller underneath the top roll, which constantly pinches the plate during rolling, and can prebend both workpiece (plate or profile) ends without removal.

finding the best plate rolling machine
finding the best plate rolling machine

Advantages 

To roll a workpiece on a 4-roll plate bending machine is extremely simple.

Introduce the plate into the roll touching it off to the opposing outboard roll for quick and accurate squaring of the plate. Then raise the lower central pinch roll and the plate is locked into position with no possibility of slipping. After this, back plate up to the near tangent point of central rolls, and then raise the left or right outboard roll to the correct position to achieve the diameter and begin rolling. When the back edge gets close, simply release the left outboard roll and bring up the right outboard roll until it touches the plate and finishes the pipe in one pass.

By comparison, a very simple operation. Because the plate is automatically squared and always pinched and not released until the pipe is complete, the net result of this difference is that the four-roll requires 66% less positioning and much less experience on the operator’s part.

In most cases, it is difficult to determine the correct position of the roll to achieve a given diameter. The operator takes his best conservative guess and moves up from there; however, given the fact that on a three-roll double pinch plate rolling machine he must have 6 positioning to achieve a diameter (even a wrong one), it becomes very time-consuming with a risk of scraping the material before correct diameter is reached. A four-roll machine, which requires only two positionings, never releases metal and arrives at the correct diameter in less than half the time, with much less risk of a scraped piece.

Given the above, there is no question the four-roll is the simplest, most productive machine available in rolling technology today.

4 roll vs 3 roll plate rolling machine

Handling the Plate

The bending light sheet presents no particular handling problem to either type of roll, although the three-roll must be lined up with a groove and then pressure applied to hold this position. On a four-roll, you merely bump the sheet off the back roll which acts as a positive stop, and then pinche the plate to insure position.

The real problems start with the rolling of a long plate. Because the three-roll pyramid has to lower one of the side rolls and pinch and pre-bend with the other, it is really not suited for long plates, as it would drag the ground. This leaves two options; the initial pinch and the four-roll.

The initial pinch can require as many as 2 or 3 people to help maintain control of the plate by using cranes, hoists, etc. Also remember, the plate has to be taken out of the machine and turned for the opposite pre-bend operation. Again, this sounds like a tough and time-consuming operation; companies doing this type of work will tell you it is tougher.

By contrast, once again, the four-roll is uniquely suited for this type of work. First, like the initial pinch, in a horizontal position allowing for conveyors or support stands, this is the safest, most controllable condition and does not require 2 or 3 men to control the plate. Secondly, the plate does not have to be turned around.

Speed

Because the initial pinch must turn the plate for the second pre-bend and the three-roll double pyramid must make 6 positioning to roll a pipe, it is conservatively estimated that the floor-to-floor time on making a pipe is 50% faster in production situations on a four-roll with much less operator expertise required. Put simply, if a three-roll can roll a vessel in 20 minutes, a four-roll could do it in 10 minutes, or twice as fast. Even if a company is rolling only a few pipes a day, there is no reason not to do them as rapidly as possible so you can get on with your other work.

Automatic Squaring of Material:

On a three-roll machine, squaring plates is a very difficult process and one of the most important. It is extremely difficult to control the squareness of the plate over a 6′- 12′ long piece with just one man.

Three-roll manufacturers usually put a small groove in the outboard rolls to help line the plate up but even with this, it often requires two men to square the plate properly. No matter how long it takes, there is no alternative; the plate has to be square or you cannot proceed. This process, on a three-roll, is time-consuming and can be very frustrating.

On a four-roll machine, the process is automatic and takes only a few seconds and, equally important, only one operator. This is done by lifting one of the independent outboard rolls and using it as a squaring gauge. Once the material is in contact all the way across, the operator simply drives the lower pinch roll up until it pinches the material and, from that point, you can roll the complete pipe in one pass.

Constantly Pinched Plate

One advantage of maintaining a pinched condition is that the operator has total control of all plate motion. In this condition, it is possible for one operator to roll parabolic curves or boxes without leaving the control and with only one squaring of the plate.

This is impossible to do on a three-roll machine. It also isn’t possible to vary the pinch pressure so that you can supply strong force for big plate and less force for thin or soft material and because the plate is driven, it prevents it from slipping out of position which happens with three-roll machines.

Another disadvantage of a three-roll double pinch machine is rolling thin sheets (less than 30% capacity) because of the lack of resistance in the material. Again, this is not a problem for a four-roll which is pinching material and creating its own drive force, regardless of resistance in the material.

CNC Controls

The four-roll machine is the only plate roll that truly utilizes a CNC control. On a three-roll machine, the CNC is basically used only to repeat side roll positioning. It cannot accurately control lateral movement. The three-roll double-pinch machine does not move the plate by driving it but rather by dragging it. There is no guarantee that the roll won’t spin slightly during movement and lose part zero.

On a four-roll machine, because of the pinching of the fourth roll, you do have constant control of the material and, therefore, the CNC control can totally control an entire bend floor to floor.

The advantages of a CNC control can be justified in either large production applications, small production applications or in “just in time situations”.

The advantage in a large production is that the computer will take care of most of the various rolling processes leaving the operator only responsible for putting the flat plate into the machine and taking the round cylinder off the machine. In cases where a vacuum feed, power feed table, and parts ejector are part of the system, the control will even put the plate on the power feed table and then feed the plate in and eject the cylinder when finished.

The control also has very strong advantages when there are a lot of different parts to be rolled even in small quantities. The control gives the operator the ability to set up the machine from one part to another in literally seconds. In either one of these situations, the value of the CNC control can not be understated.

Works Cited